Seismometer



P 1931- w. o. SNELLING ET AL 5,554

SEISMOMETER Filed Nov. 12, 1925 Nlllllllll I h Patented Sept. 29, 1931 UNITED STATES PATENT OFFICE WALTER 0. SNELLING AND GUY A. EUPP, OF ALLENTOWN, PENNSYLVANIA, ASSIGNORS 1'0 TROJAN POWDER COMPANY, OF NEW YORK, N. Y., A CORPORATION OF NEW YORK SEISMOMETER Application filed November 12, 1925. Serial No. 68,594.

The principal object of this invention is to provide a seismometer well adapted to measure the vibration producing in blasting. More specifically, the invention resides in the provision of a seismometer so constructed that the active indicating elements are not affected by the violeE air wave produced by the blast.

The invention further resides in certain novel and highly advantageous features of construction facilitating the operation of setting up the apparatus, as hereinafter fully set forth and as illustrated in the attached drawings, in which:

Figure 1 is a plan View of the apparatus, and

Fig. 2 is a side elevation.

With reference to the drawings, our device may consist in a preferred form of a '20 suitable fiat bottomed container 1 divided in the interior by partitions 2 into a number of independent compartments 3. For this container, a coverplate 4 is provided preferably of plate glass or other suitable transparent material which may be held in place by means of clamps 5, as clearly illustrated.

The device further comprises a plurality of cylindrical pins 6 of varying diameters, these pins corresponding in number to the compartments 3 and being established in the respective compartments as illustrated. Instead of employing pins of the same height and different diameters, the same results may be obtained by employing pins of the same diameter but of varying heights, or both diameter and height may be varied if desired.

The device further comprises a base plate 7 upon which is mounted a suitable leveling head 8 which supports the box 1. The boX 1 is preferably pivotally secured to the head 8 to permit horizontal rotation, and a set screw 9 is provided for securing the box rigidly to the head in adjusted position. As illustrated in Fig. 2, the head 8 comprises a lower part 10 threaded on the base plate 7 and an upper part supported on the part 10 by means of a plurality of screws 11 adjustable in the head and constituting pedes- 50 tals or legs for the said upper part. In the present instance, we provide four of these screws and each is provided with a knurled flange 12 to facilitate their adjustment to bring the box 1 to a level position.

The level position is determined in the present instance by means of two leveling bulbs 13 and 1 1 of well known type which may be mounted in the floor of the container 1 and as shown in Fig. 1 at right angles to each other, thereby permitting active adjustment of the container through the leveling head to a level or truly horizontal position.

In practice, the base plate is securely fastened to pins driven into the ground or to suitable expansion bolts in holes drilled in rock. With the pins 6 set on end in their respective compartments, the screws 11 of the leveling head 8 are adjusted to bring the floor of the box 1 upon which the pins rest into an accurately level position. The plate glass cover 4 makes it possible to note the condition of the pins at any time without opening the apparatus and to verify the fact that the pins are all upright after the final leveling and adjusting operation.

While it has been known that a series of vertical pins of varying heights and diameters may be employed as a means of measuring the intensity of earthquake shocks, efforts to apply this principle to the measurement of earth movement or vibration produced in blasting have been unsuccessful. We have found that this has been due to the effects of the powerful air wave produced at the moment of the explosion, which air wave has a greater overturning effect upon vertical pins than the earth movement produced by the explosion. We have discovered that by placing a series of pins of gradually increasing diameter and of the same height within a series of airtight compartments so as to subject the pins to the action of the earth movement alone by preventing the air wave from having the aforesaid overturning effect, we are able to obtain a satisfactory measurement of the ground movement produced by blasting.

In constructing apparatus in accordance with the present invention, the drawings forming part of the application may be considered as working drawings and may be followed exactly as to all dimensions concerned. The box or housing may be made of any suitable material, and in the construction of commercial units brass, steel and aluminum have been alternatively employed in the construction of the box or housing. In view of the fact that this apparatus must be used in the field, and occasionally has to be carried for considerable distances, sheet aluminum forms a very satisfactory material of construction, because of its relatively light weight, as compared with either brass or steel.

The pins may also be made of aluminum, brass or steel, or other suitable material, steel being preferable because of its superior hardness. Although aluminum and brass pins have been used successfully, there is a tendency for them to become somewhat marred or indented from falling, and any irregularity of the base of the pin is of course harmful, as it interferes with the stability of the pin, tending to make the pin some what more unstable than it would be if perfect and unmarred.

Although the drawings forming part of the present application may be followed exactly as to both scale and dimensions, it is to be noted that changing the scale has relatively little effect upon the operation of the apparatus, and at the present time the seismometer units that are being made and used commercially are in most cases twice the scale shown in the drawings, the boxes being for example ten inches square, and the pins being four inches in height.

It is desirable theoretically to establish a strictly mathematically related series of pin lengths and pin diameters such that each pin will represent a uniform gradation in stability as compared with the next pin to it in the series, although this is not essential to a useful application of the invention. In units constructed in accordance with the invention, pin heights of two inches and of four inches have been successfully employed, with pin diameters of 2, 3, 4, 5, 6, 7, 8, 9, and 10 millimeters and of 4, 6, 8, 10, 12, 14, 16, 18 and 20 millimeters respectively. In other instances pins varying from in. to 1 in. in diameter and from 2 in. to 24 in. in height have been used. Where the diameter of the pin is constant and the height is varied, pins of a standard diameter of A in., and of heights of 2, 4, 6, 8, 10, 12, 14, 16 and 18 in., for apparatus using nine pins, have been used. The exact number of pins employed is of relatively little significance, although at least six pins should be used, in order to obtain a sufficient range of measurement, and the use of more than 16 pins makes the work of setting upthe apparatus between successive blasts somewhat slow.

The relationship between the size of a blast and the distance at which the apparatus is set up is one which from the very nature of the problem can only be stated in the broadest general terms. A principal use of this apparatus is to determine whether or not buildings located at a distance from quarries or other points at which blasting is done are subjected to sufficient shock from such blasting to cause structural damage to the buildings. The results are, therefore, subject to the interpretation of men who are particularly familiar with the very complicated problem of determining the relationship between the amount of vibration existing in buildings, due to normal conditions and use, and the vibration or movement produced by blasting. With charges of explosives of less than one ton, in well drilled holes properly placed, damage to buildings is rarely alleged at a distance of more than 1,000 ft., although in the construction of subways in New York city there were some cases where tests were made involving quantities of explosive as small as 100 pounds, and distances as short as 100 ft.

Present day quarry blasts, of the type in conjunction with which the present apparatus is normally used, vary from 10,000 pounds of explosive to 100,000 pounds of explosive, and with blasts of this magnitude the apparatus is usually set up in buildings that are located from one-fourth mile to one mile from the point at which the blast is made. There none of the pins are overturned, the conclusion has usually been drawn that the vibration has been of too small magnitude to cause structural damage to the building in which the tests are being made, since parallel tests made by moving furniture in such a building, or even by a man walking across the floor of the room in which the tests are made sometimes results in overturning two or three of the pins of smallest diameter or of greatest height, ac- 1 cording to the type of pins that are used.

In order to more fully explain the exact use of the present apparatus, the following abstract from notes made in connection with tests made under the direction of the court in one important legal case in which structural damage to a residence from blasting in an adjacent quarry was claimed, is hereby offered:

The apparatus used was a seismometer according to the present invention having an aluminum case 8 in. x 8 in. x 4% in., with a transparent plate glass top and containing nine pins having a standard height of 100 mm., and having the following diameters 5, 6, 7, 8, 9, 10, 11, 12 and 14 mm.

This apparatus was set up successively in a number of buildings, located at distances of from 1,200 ft. to 3,800 ft. from the portlon of the quarry face Where'blasting was being conducted. A total of 31 individual tests were made, involving separate charges of explosive varying from 900 lbs. of 40% dynamite to 30,000 lbs. of 60% dynamite. In many of the tests none of the pins were overturned, and in none of the tests were more than three pins overturned. A man weighing 170 lbs. walking rapidly across the floor of the room on which the apparatus was standing caused the overturning of four of the pins, and a heavy truck loaded with stone passing rapidly on the highway in front of one of the buildings caused the overturning of two of the pins.

These results gave a measurement of the approximate vibration produced within the buildings, not in absolute terms of acceleration as might be obtained from the more usual type of seismometers, but in relative terms, of a type that experience has shown can be readily understood by jurors, and can be readily comprehended by the various parties to any controversy relating to alleged damage from blasting. In at least one legal case apparatus of the present type was used in court, in conjunction with a highly refined and exceptionally delicate portable seismograph, the results obtained by the seismograph being of so technical a nature that they were not understood by anyone except perhaps one or two of the experts present, while the results obtained with the seismometer made in accordance with the plans of the present inventors was understood by all present, and led to the very prompt clearing away of many of the difficulties which had stood in the way of a settlement of the case.

The relationships which exist between the dimensions of a pin and the conditions necessary for its overturning by accelerated motion have been stated by a prominent scientist as follows:

VV= weight of body in lb.

9 acceleration due to force of gravity 32.2 ft. per sec. per sec.=386.4 in. per sec. per see.

a acceleration in inches per sec. per sec.

'0" velocity in in. per sec.

A single amplitude in inches n= frequency=number of vibrations per second then F=Z?@@=% a, and kinetic energy or determined from the above, using the following conventional abbreviations:

W= total weight of pin in pounds to weight of pin per linear inch h height of pin in inches (Z diameter of pin in inches '0 maximum velocity of pin during vibration in inches per second g= acceleration due to gravity=322 ft. per sec. per sec.=386.4t in. per sec. per sec. a= maximum acceleration of pin during vibration in inches per sec. per see. A maximum single amplitude of motion in inches Employing the above abbreviations we may now express the work in terms of inch pounds necessary to overturn a pin by the following approximate formula:

It is true that the above is not an absolutely definite relationship, the exact expression involving some additional factors which materially complicate the problem, and which are not involved in any practical way in the overturning of any pin whose height is several times its diameter, as is the case with all pins used in applicants seismometers. Since the present approximate formula gives results which are extremely close to the results given by the complete formula, and involves no appreciable errors, it has been concluded that this approximate formula is entirely satisfactory for all pins within the limits of dimensions used in applicants seismometers, these limits being for example, from 2 mm. in diameter to 25 mm. in diameter, and from 50 mm. in height to 400 mm. in height, the height of the pin in every case being more than ten times its diameter. Within these limits, the kinetic energy set up in the pin by motion is 2 5;- in which W= wk, and

o aA Wo wd Y T who tool 29 w T 2 gdz i7;

hence from which 193.203 T an overturning of the pins, and in practical use the behavior of the pins is usually observed through the transparent cover, and the number of pins which vibrate or wabble, but are still not overturned, is determined as well as the number of pins which are overturned in the test. Frequently a minor displacement of the recording element, but within the limit of its ability to restore equilibrium, is a significant, and helpful factor in connection with the study of vibrational effects, and particularly when considered in conjunction with the overturning of one or more of the pins or lesser stability.

While the herein described apparatus has been made in accordance with the foregoing principles and provides a compact, easily manipulated and accurate measuring device of the type set forth, it will be understood that the device is capable of considerable modification without departurefrom the essential features of the invention.

We claim:

1. In a seismometer, a housing having therein partitions forming a series of segregated compartments, eachof said compartments having a level floor, a cylindrical pin in each compartmentfvertically standing on said floor, all of said pins having one of the two dimensions of length and diameter in common and each pin varying from every other pin in the other of 'said dimensions, said pins thereby forming a progressive series with respect toone of said:

dimensions, while being thelsame with respect to the other of said dimensions.

2. In a seismometer, a houslng having therein partitions forming a series of seg-- regatedrcompartments, each of said c0m partments having a level floor, a transparent cover for said compartments, a cylindrical pin in each compartment vertically standing on said floor, all of said pins having one of the two dimensions of length and diameter in common and each pin varying from every other pin in the other of said dimensions, said pins thereby forming a progressive series in relation to oneof said dimensions, while being the same with respect to the other of said dimensions.

diameter as every other pin of the series and each pin differing in the other of said dimensions from every other pin of the series,

said pins forming a uniformly progressive series with respect to one of the said dimensions, while all being the same with respect to the other of said dimensions.

5. A seismometer comprising a substantially air-tight container, and a series of cylindrical pins adapted for mounting therein, each of said pins diflering from the others in at least one of the two dimensions of length and diameter so as to form a series, the component members of which vary progressively as to stability in the vertically standing positions.

6. In a seismometer, a housing having therein partitions forming. a series of segregated compartments, each of said compartments having a level floor, a cylindrical pin in each compartment vertically standingcon said floor, each of said pins differing fromthe others in at 'least one of the two dimensions of length and diameter so as to formaseries, the component members of which vary progressively'as to stability in the said vertically standingpositions.

-7. In. a seismometer, a housing having therein partitions forming a series of segregated compartments, each of said compartments having a level floor, and a vertically disposed cylindrical .pin in each compartment resting upon said floor, all of said pins having the same length but each pin varying from every other'pin indiameter, said pins forming a progressive series in relation to their diameters.

WALTER O. SNELLING. GUY A. RUPP.

3. In a seismometer, a housing having 7 therein partitions forming a series of seg-' regated compartments, each of said compartments having a level floor, and a vertically disposed cylindrical pin in each com partment resting upon said floor, all of said pins having the same diameter but each pin varying from every other pin in length, said pins forming a progressive series in relation to their length.

4. A seismometer comprising a substantially air-tight container, and a series of cylindrical pins adapted for mounting in said container, each of said pins being the same in one of the two dimensions of length and 

